Nikon D600 dust / oil

I purchased the d600 in june 13 from john lewis thinking as it was not one of it,s first production and therefore should not suffer from dust marks.

In october after a trip to skye lots of my shots required lots of p.s. that I put down to marks on the lens. Later i proved it was not the lens and was the sensor and have since took it back to john Lewis who have quoted me 4-6 weeks.

I am now thinking john lewis sold me a camera that was not fit for purpose. Furthermore proved by nikon addressing the issue by bringing out the d610, the same camera with a new shutter to rectify oil and dust problems caused by the shutter in the D600.

I am interested in your replys to weather I have good grounds for replacement/refund?